This invention relates to a reciprocal furnace hearth for heating metallic parts for further processing, such as hot-forming the parts into any desired form, such as hole saw blades of cylindrical configuration.
It has generally been the practice in the past to fabricate tools, such as hole saws, by individually heating flat saw blades in an open gas flame. Even at the present time, this open hearth technique still persists for making hole saws larger than three inches in diameter. The open hearth process, including timing of the heating cycle, is manually controlled by the individual operator whereby the quality of the hole saws produced is dependent on the skill and diligence of the operator. Another drawback of the open hearth furnace, resulting from the oxygen supported combustion employed in the open hearth, is extensive scaling of the hole saw blades which requires grit blasting to remove the scale after the hot-forming operation.
It had also been proposed to heat metallic parts in a linear furnace in which the parts are carried through the furnace on a continuous link-chain conveyor. This system was not considered feasible, however, because of the inability of the chain feed to withstand the high operating furnace temperatures, in excess of 2,000 degrees Fahrenheit. One proposed solution was disclosed in U.S. Pat. No. 3,656,720 for a HEAT TREATING FURNACE WITH WALKING BEAM DRIVE.
Very recently, the rotary furnace has found acceptance for high quality, small diameter, hole saws in which saw blade blanks are electrically heated in an inert atmosphere. Saw blades are intermittently inserted at one position into a rotatable ceramic hearth within the furnace and removed after predetermined angular rotation. The loading and unloading steps are continuous and upon removal, each blade is hot-formed on a mandrel to the cylindrical configuration required for hole saws. While the rotatable furnace has been successfully utilized for fabricating hole saws, it was found practicable for small hole saws of less than three inches in diameter.
While the rotary furnace is suitable in making smaller size hole saws, it is not practicable for larger size hole saws because the overall diameter of the furnace and hearth would have to be increased by a factor of two times the increase in hole saw diameter. Thus, for every one inch increase in hole saw diameter, it would mean a 2.pi. or 6.28 inch increase in the size of the rotary furnace whereby a six inch hole saw would require a ceramic hearth having a diameter almost nineteen inches larger than the diameter of the hearth used for a three inch hole saw. A ceramic hearth of such large size would have a tendency to crack in such high temperature applications and would require frequent replacement.
There are a number of prior United States patents which disclose a reciprocable mechanism for batch heating of various work pieces in furnaces, including U.S. Pat. Nos. 1,280,037; 1,741,624; 2,842,352; 3,563,434; 3,744,650 and 4,695,706. U.S. Pat. No. 2,600,401 discloses a reciprocating hearth mechanism which depends upon inertia to slip and slide work pieces along the surface of a reciprocal hearth. U.S. Pat. No. 3,656,720 discloses a heat-treating furnace with a walking beam assembly for moving work pieces through a high temperature zone. None of these patents, however, discloses a reciprocal hearth which is sequentially loaded and unloaded within the high temperature zone of the furnace per se.
It is the principal object of this invention to provide a reciprocal furnace hearth for heating metallic parts to a predetermined temperature for further processing and which overcomes the limitations of all the previously available types of furnaces, including the rotary hearth type adaptable for a similar purpose.